1. Field of the Invention
The present invention relates to an insert for an indexable insert drill attached to an insert detachable drill, and an indexable insert drill to which the insert for an indexable insert drill is attached. Priority is claimed on Japanese Patent Application No. 2008-089679, filed Mar. 31, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
As an insert for a drill and an indexable insert drill, a drill in which substantially square flat plate-shaped inserts for a drill are detachably attached to the tip of a drill body on the radial inner peripheral side and outer peripheral side which are opposite to each other across the axis of the drill body is disclosed in, for example, Japanese Patent Examined Patent Application, Second Publication No. Sho 53-15234 and Published Japanese Translation No. 2005-527390 of PCT International Publication. In such an indexable insert drill, one of the cutting edges formed at respective sides of a square rake face of the insert for a drill is made to protrude toward the tip of the drill body. The single cutting edges of the inner and outer peripheral inserts for a drill are made to intersect each other in a rotational locus around the axis, the inner peripheral side of a machining hole is drilled by the radial inner peripheral cutting edge, and the outer peripheral side of the machining hole is drilled by the single cutting edge of the outer peripheral insert, respectively, whereby the machining hole with a predetermined diameter is formed in a work material.
However, in an indexable insert drill described in Japanese Patent Examined Patent Application, Second Publication No. Sho 53-15234 among the above publications, both the inner and outer peripheral inserts for a drill are attached such that a corner cutting edge located at a corner of a square rake face is made to protrude toward the distal end of a drill body in the direction of the axis of the drill body, and the single cutting edge of each insert for a drill contacts a work material from the corner cutting edge. Therefore, loss of the cutting edge may easily occur. Additionally, in the radial outer peripheral insert of these inserts, a corner cutting edge between the single cutting edge and the other single cutting edge adjacent to the outer peripheral side of this single cutting edge is located at an outermost periphery of the drill body. Therefore, when the amount of feed is increased, there is also a possibility that the surface quality of the inner wall surface of a machining hole may deteriorate.
Meanwhile, in the insert for a drill described in Published Japanese Translation No. 2005-527390 of PCT International Publication, a transition cutting edge is provided in the above single cutting edge and formed in the shape of a step. In this case, when such a cutting edge is directed toward the outer peripheral side of the drill body in the radial outer peripheral insert for a drill, there is a possibility that the step may interfere with the inner wall surface of a machining hole, and degradation of surface quality may also occur. Additionally, in the indexable insert drill described in Published Japanese Translation No. 2005-527390 f PCT International Publication, a cutting edge portion located at an extreme end in the direction of the axis of the drill body in the above single cutting edge is formed in the shape of a straight line extending in a direction substantially vertical to the axis of the drill body. Therefore, contact of the cutting edge portion to a work material is performed simultaneously. Thus, a large impact occurs, and balance is easily upset. Moreover, inserts for a drill differ on the inner peripheral side and outer peripheral side. Therefore, there is also a possibility that management may become complicated, and erroneous attachment of the inserts may occur at the inner and outer peripheries.
The invention was made in light of the above, and has an object to provide an insert for a drill capable of providing a high surface quality of the inner wall surface of a machining hole, extending the lifespan of an insert, and improving machining accuracy (cutting accuracy) without causing a loss or radial run out of cutting edges at the time of contact with a work material, and to provide an indexable insert drill capable of performing such drilling using the same insert on the inner and outer peripheral sides.